Fix #18145 and also avoid needless work with implicit vars

- `forAllOrNothing` now is monadic, so we can trace whether we bind
an explicit `forall` or not.
- #18145 arose because the free vars calculation was needlessly
complex. It is now greatly simplified.
- Replaced some other implicit var code with `filterFreeVarsToBind`.
Co-authored-by: Ryan Scott <ryan.gl.scott@gmail.com>